102 THE VEGETATIVE FUNCTIONS OF PLANTS 
the restricted supply of oxygen that can be obtained in 
the absence of air. // the oxidation processes could be 
completed the end products would be carbon dioxide and 
water. Organisms that may continue to live without a 
supply of free oxygen are called anaerobes. 1 Either the 
entire organism may live anaerobically, or this condition 
may be confined to one or more of its organs, or even to 
a single cell or group of cells. It is possible for yeast to 
live in contact with free oxygen, but when this is not 
present it can secure sufficient oxygen for its needs by 
alcoholic fermentation alone. 2 
104. Relation of Fermentation to Our Daily Lives. 
Reference has already been made to the fact that such 
diverse operations as the manufacture of all alcoholic 
drinks and the making of bread are dependent upon 
fermentations, and we have seen that the process funda- 
mental to all life the formation of carbohydrates by 
photosynthesis probably involves the action of at least 
six different ferments. The active principles of all the 
digestive juices of our own bodies are also enzymes 
diastase in the saliva, pepsin in the stomach, trypsin in the 
intestine, and so on. We shall learn, in the next chapter, 
that all muscular and mental activity, even life itself, is 
dependent on the fermentative activity of enzymes. 
1 a + aer (air) + bios (life) = living without air. 
2 From a reading of Chapters VII to IX the student will learn that 
the transformations wrought in other substances by the catalytic agents, 
called enzymes, are not all of the same nature; some (as alcoholic fer- 
mentation, and digestive processes) are destructive, others (as the poly- 
merizations and other transformations in photosynthesis, mentioned on 
pages 79 and 80) are constructive. The author believes that it con- 
tributes to simplicity, without sacrificing clear and accurate thinking, to 
consider " enzyme " and " ferment " as synonymous terms, and to call all 
activities of enzymes fermentations, whether they are analytic or synthetic. 
